Light olefin (e.g., C4− olefin) can be used to make a wide range of useful products. For example, ethylene and/or propylene can be polymerized to produce polymer, such as polyethylene, polypropylene, ethylene-propylene copolymer, etc. Hydrocarbon pyrolysis (e.g., steam cracking) is one common way to produce light olefin.
Besides light olefin, steam cracker effluent typically contains molecules boiling in the naphtha boiling-range (hereinafter “steam cracker naphtha” or “SCN”). Steam cracker naphtha comprises a mixture of compounds (including olefins) having an initial atmospheric boiling point in the range of about 25° F. (−3.9° C.) to about 35° F. (1.7° C.) and a final atmospheric boiling point in the range of about 430° F. (221° C.) to about 550° F. (288° C.). Steam cracker effluent also generally contains compounds having atmospheric boiling points in the gas oil boiling range (hereinafter “steam cracker gas oil”, or “SCGO”). Like SCN, SCGO comprises a mixture of compounds, primarily a mixture of hydrocarbon compounds. Although there is typically an overlap between SCN and SCGO in composition and boiling point range, SCGO typically has an initial atmospheric boiling point that is approximately the same as or greater than the SCN's final atmospheric boiling point. The SCGO's final atmospheric boiling point is typically about 1050° F. (566° C.). Steam cracker effluent can also contain steam cracker tar having an atmospheric boiling point >about 1050° F. (566° C.).
It is conventional to cool steam cracker effluent by directly or indirectly contacting the effluent with a quench medium such as quench oil. Effluent cooling leads to condensation into the liquid phase of at least a portion of the SCGO. The SCN and light olefin typically remain in the vapor phase after effluent cooling, so that the primarily liquid-phase phase SCGO can be separated and conducted away. This separation is typically carried out in a primary fractionator.
Generally, at least three streams are conducted away from the primary fractionator: (i) a vapor stream comprising molecular hydrogen, light hydrocarbon (including light olefin), and SCN; (ii) a liquid stream comprising SCGO; and (iii) a tar stream comprising steam cracker tar. Typically, the primary fractionator includes a rectification region and a stripping region. The rectification region concentrates into the vapor phase those components of the steam cracker effluent having greater volatility, e.g., SCN and light olefin. The stripping region concentrates into the liquid phase those components of the steam cracker effluent having lesser volatility, e.g., SCGO. Some primary fractionators include a collection/distribution region located between the stripping and rectification regions for collecting and distributing into the stripping region liquid that is disengaged from vapor in the rectification region.
The separated vapor conducted away from the primary fractionator's rectification region typically comprises molecular hydrogen, C4− hydrocarbon (including light olefin), and SCN. Additional quenching stages can be used for condensing at least a portion of the SCN in the separated vapor. The condensed SCN and quench water are typically recovered from the C4− hydrocarbon vapor in a separation stage, which can include, e.g., one or more flash drums. The additional quenching stage can be carried out in a quenching vessel, an exchanger, or in a third region (a quenching region) of the primary fractionator, the quenching region being located above the primary fractionator's rectification region. The molecular hydrogen and C4− hydrocarbon (including the light olefin) are conducted from the flash drum as a vapor-phase (e.g., as flash-drum vapor) to one or more recovery stages for recovering one or more the desired light olefin (e.g., ethylene and/or propylene). These can be stored and/or subjected to further processing, such as polymerization. Separated SCN and quench water are conducted away from the flash drum, typically as flash drum bottoms. Quench water is typically separated from the SCN by gravity separation, e.g., in a settling vessel.
The separated SCN (primarily in the liquid phase) is typically divided into two streams of substantially the same composition. The first SCN stream is recycled to the fractionator (e.g., as reflux). The second SCN stream is typically subjected to further processing to produce motor gasoline and motor gasoline blending components. An aqueous stream is also typically recovered, for example, when the quenching includes directly contacting the steam cracker effluent with water.
One common primary fractionator generally has the form of a substantially-cylindrical vessel with a greater diameter in the stripping region (the vessel's lower region) and a lesser diameter in the rectification region (the vessel's upper region). The cylindrical vessel's long axis is typically perpendicular to the surface of the earth. When oriented this way, liquid SCN reflux results in additional downflow of liquid in the rectification region of the fractionator vessel, leading to additional cooling of upflowing vapor-phase components. This in turn leads to increased condensation into the liquid phase, which increases fractionator efficiency. The increased fractionator efficiency provides greater purity in separated vapor and liquid phases conducted away from the fractionator.
It has been reported that liquid SCN reflux comprises foulant precursors such as styrene, indene, dicyclopentadiene, divinylbenzene, decalin, tetralin, naphthalene, and alkylated derivatives thereof. See, e.g., Performance Evaluation and Fouling Mitigation in a Gasoline Fractionator, M. Sprague, et al., Proceedings of AIChE Spring National Meeting, Orlando Fla., (2006). The reference discloses that fractionator fouling results primarily from maldistribution of liquid SCN reflux in the rectification region. The liquid maldistribution leads to short-circuiting of the reflux to higher-temperature regions of the tower. This results in less efficient disengagement of the vapor in the fractionator and also increased foulant polymerization and accumulation. Foulant accumulation was controlled by lessening the amount of reflux short-circuiting and by utilizing certain stainless steels in fractionator locations that are particularly prone to fouling. Should the foulant precursors accumulate to an amount that cannot be mitigated by these methods, the SCN reflux's foulant content can be lessened by periodically purging the reflux loop, e.g., by periodically increasing the relative volumetric flow of the second SCN stream and decreasing the relative volumetric flow of the first SCN (reflux) stream.
More recently, it has become desirable to produce light olefin by steam cracking relatively low molecular weight feeds such as ethane. As reported in Reduce Fouling & Corrosion Risks and Improved Reliability While Transiting to Mixed Feed Operations, M. Jain et al., Proceedings of AIChE Spring National Meeting, San Antonio, Fla., (2013), steam cracking light gases produces more light olefin but less SCN and SCGO than does the steam cracking of liquid hydrocarbon feeds. Consequently, increasing the amount of light gases in the steam cracker feed results in an increase in vapor volumetric flow rate relative to liquid in the fractionator, and an increase in the amount of foulant precursors in the first (reflux) SCN stream and the second SCN stream. The increased amount of foulant precursors in the SCN causes additional fractionator fouling and an undesirable increase in naphtha mass density. In pyrolysis systems where the quenching of the steam cracker effluent includes a direct quench of the SCN with quench water, the increased naphtha density and the increased amount of foulant precursor in the SCN also increases emulsification of the naphtha-quench water mixture, which decreases naphtha separation efficiency, leading to a further increase in fractionator fouling.
The Jain reference discloses that these difficulties can be at least partially overcome using chemical antifouling agents and by introducing gasoline from an external source into the primary fractionator (e.g., via the SCN reflux stream). Another conventional way to overcome this difficulty is to periodically increase the relative amount of liquid hydrocarbon in the steam cracker feed under substantially constant steam cracking conditions, while decreasing or maintaining substantially constant the volumetric flow rate of the second naphtha stream. The increased amount of liquid hydrocarbon in the steam cracker feed is then maintained, at least until the desired volumetric flow rate is achieved in the SCN reflux loop. These conventional methods are undesirable for several reasons. First, the use of antifouling agents and gasoline from an external source is inefficient and costly. Moreover, modifying the steam cracker feed system to allow for the periodic introduction of liquid hydrocarbon feed increases the complexity of feed system piping and leads to unfavorable process economics particularly when the marginal cost of liquid hydrocarbon feeds significantly exceeds that of gaseous feeds.
It is therefore desired to increase the amount of gaseous hydrocarbon in the steam cracker feed while lessening or eliminating the need for (i) antifouling agents, (ii) an external naphtha source, (iii) periodic purging of the SCN reflux loop, and (iv) periodically increasing the amount of liquid hydrocarbon in the steam cracker feed.